Beamformed downlink communications for a multiple antenna system

ABSTRACT

Some of the example embodiments presented herein are directed towards an eNodeB ( 401 ), and corresponding method therein, for establishing beamforming for downlink communications in a multiple antenna system. The eNodeB ( 401 ) may be configured to transmit a plurality of reference signals, where each reference signal is beamformed into a distinct direction with in at least one correlated domain (e.g., an elevation and/or azimuth domain). The eNodeB ( 401 ) may thereafter generate beamformed downlink communications for antenna elements and/or subelements based on received signal quality assessments of the plurality of reference signals. 
     Some example embodiments may be directed towards a user equipment ( 505 ), and corresponding methods therein, for establishing beamforming for downlink communications. The user equipment ( 505 ) may be configured to receive the plurality of reference signals and provide signal assessments of the reference signals based on measurements performed by the user equipment ( 505 ). The user equipment ( 505 ) may thereafter transmit the signal quality assessments to the eNodeB ( 401 ) and receive beamformed downlink communications based on the signal quality assessments.

TECHNICAL FIELD

Example embodiments are presented herein for establishing beamformingfor downlink communications in a multiple antenna system

BACKGROUND Overview of Multi-Antenna Systems

Multi-antenna techniques may be used to significantly increase the datarates and reliability of a wireless communication system. Systemperformance may in particular be improved if both the transmitter andthe receiver are equipped with multiple antennas, which results in amultiple-input multiple-output (MIMO) communication channel. Suchsystems and/or related techniques are commonly referred to as MIMO.

The LTE standard is currently evolving with enhanced MIMO support. Acore component in LTE is the support of MIMO antenna deployments andMIMO related techniques. For instance there is LTE-Advanced support fora spatial multiplexing mode with the possibly channel dependentprecoding. Precoding is a form of beamforming to support multi-layertransmission in multi-antenna wireless communications. Beamforming is asignal processing technique used in sensor arrays for directional signaltransmission or reception.

Spatial multiplexing is transmission techniques in MIMO wirelesscommunications to transmit independent and separately encoded datasignals from each of the multiple transmit antennas. The spatialmultiplexing mode is aimed for high data rates in favorable channelconditions. An illustration of the spatial multiplexing operation isprovided in FIG. 1.

As seen FIG. 1, the information carrying symbol vector s is multipliedby an N_(T)×r precoder matrix W_(N) _(T) _(×r), which serves todistribute the transmit energy in a subspace of the N_(T) (correspondingto N_(T) antenna ports) dimensional vector space. The precoder matrix istypically selected from a codebook of possible precoder matrices, andtypically indicated by means of a precoder matrix indicator (PMI), whichspecifies a unique precoder matrix in the codebook for a given number ofsymbol streams. The r symbols in s each correspond to a layer and r isreferred to as the transmission rank. In this way, spatial multiplexingis achieved since multiple symbols can be transmitted simultaneouslyover the same time/frequency resource element (TFRE). The number ofsymbols r is typically adapted to suit the current channel properties.

LTE uses OFDM in the downlink (and DFT precoded OFDM in the uplink) andhence the received N_(R)×1 vector y_(n) for a certain TFRE on subcarriern (or alternatively data TFRE number n) is thus modeled by:

y _(n) =H _(n) W _(N) _(T) _(×r) s _(n) +e _(n)

where e_(n) is a noise/interference vector obtained as realizations of arandom process. The precoder, W_(N) _(T) _(×r), may be a widebandprecoder, which is constant over frequency, or frequency selective. Notethat the signals above (e.g., y_(n)) could alternatively represent asignal in the time-domain. It is generally understood that signalsdescribed herein may represent signals in other domains than in thetime-frequency grid of an OFDM system.

The precoder matrix is often chosen to match the characteristics of theN_(R)×N_(T) MIMO channel matrix H, resulting in so-called channeldependent precoding. This is also commonly referred to as closed-loopprecoding and generally strives for focusing the transmit energy into asubspace which is strong in the sense of conveying much of thetransmitted energy to the user equipment. In addition, the precodermatrix may also be selected to strive for orthogonalizing the channel,meaning that after proper linear equalization at the user equipment theinter-layer interference is reduced.

In closed-loop precoding for the LTE downlink, the user equipmenttransmits, based on channel measurements in the forward link (downlink),recommendations to the eNodeB of a suitable precoder to use. The userequipment selects a precoder out of a countable and finite set ofprecoder alternatives, referred to as a precoder codebook. A singleprecoder that is supposed to cover a large bandwidth (widebandprecoding) may be fed back. It may also be beneficial to match thefrequency variations of the channel and feedback a frequency-selectiveprecoding report, e.g., several precoders, one per sub-band. This is anexample of the more general case of channel state information (CSI)feedback, which also encompasses feeding back other entities thanprecoders to assist the eNodeB in subsequent transmissions to the userequipment. Such other information may comprise channel qualityindicators (CQIs) as well as a transmission rank indicator (RI). For theLTE uplink, the use of closed-loop precoding means the eNodeB isselecting precoder(s) and transmission rank and thereafter signals theselected precoder that the user equipment is supposed to use.

SUMMARY

Active antennas may comprise many subelements and arrays of activeantennas may comprise even more. Such antenna configurations wereneither thought of, nor taken into account, when existing codebookswhere designed. Thus, at least one example object of the exampleembodiments presented herein is to provide spatial multiplexingtransmission techniques in MIMO wireless communications which accountfor the various subelements which may be comprised in active antennaarrays. At least one example advantage of such a system is the abilityto provide user equipment specific beamforming in an efficient manner.Specifically, user equipment specific beamforming in an azimuth and/orelevation direction may be achieved without the need of excessivefeedback overhead from the user equipment. Another example advantage maybe the reduction of interference within the network and improved signalquality.

Thus, some of the example embodiments are directed towards a method, inan eNodeB, for establishing beamforming for a multiple antenna system.The eNodeB is comprised in a wireless communications network. The methodcomprises transmitting, to a user equipment, a plurality of referencesignals. Each reference signal is beamformed into a distinct directionwithin at least one correlated domain of the multiple antenna system.The method further comprises receiving, from the user equipment, signalquality assessments for each of the plurality of reference signals. Themethod also comprises generating downlink communication signals forantenna elements and/or sub-elements of the multiple antenna system. Thedownlink communication signals are beamformed in a transmittingdirection in the at least one correlated domain. The transmittingdirection is determined, in-part, from the signal quality assessments.

Some of the example embodiments are directed towards an eNodeB forestablishing beamforming for a multiple antenna system. The eNodeB iscomprised in a wireless communications network. The eNodeB comprisesradio circuitry configured to transmit, to a user equipment, a pluralityof reference signals. Each reference signal is beamformed into adistinct direction within at least one correlated domain of the multipleantenna system. The radio circuitry is further configured to receive,from the user equipment, signal quality assessments for each of theplurality of reference signals. The eNodeB further comprises processingcircuitry configured to generate downlink communication signals forantenna elements and/or sub-elements of the multiple antenna system. Thedownlink communication signals are beamformed in a transmittingdirection in the last one correlated domain. The transmitting directionis determined, in-part, from the signal quality assessments.

Some of the example embodiments are directed towards a method, in a userequipment, for establishing beamforming for a multiple antenna system.The user equipment is comprised in a wireless communications network.The method comprises receiving, from an eNodeB, a plurality of referencesignals. Each reference signal is beamformed into a distinct directionwithin at least one correlated domain of the multiple antenna system.The method also comprises transmitting, to the eNodeB, signal qualityassessments of each of the plurality of reference signals. The methodfurther comprises receiving, from the eNodeB, downlink communications.The downlink communications are beamformed in a direction in the atleast one correlated domain. The receiving direction is determined,in-part, from the signal quality assessments.

Some of the example embodiments are directed towards a user equipmentfor establishing beamforming for a multiple antenna system. The userequipment is comprised in a wireless communications network. The userequipment comprises radio circuitry configured to receive, from aneNodeB, a plurality of reference signals. Each reference signal isbeamformed into a distinct direction within at least one correlateddomain of the multiple antenna system. The radio circuitry is furtherconfigured to transmit, to the eNodeB, signal quality assessments ofeach of the plurality of reference signals. The radio circuitry is alsofurther configured to receive, from the eNodeB, downlink communications.The downlink communications are beamformed in a receiving direction inthe last one correlated domain. The receiving direction is determined,in-part, from the signal quality assessments.

DEFINITIONS 3GPP 3rd Generation Partnership Project CoMP CoordinatedMulti Point CRS Common Reference Symbols CSI Channel State InformationCQI Channel Quality Indicator DFT Discrete Fourier Transform DL DownlinkeNB Evolve Node B EPRE Energy Per Resource Element FDD FrequencyDivision Duplexing

GSM Global System for Mobile communications

LTE Long Term Evolution MIMO Multiple Input Multiple Output OFDMOrthogonal Frequency-Division Multiplexing PUSCH Physical Uplink SharedChannel PMI Precoder Matrix Indicator RAN Radio Access Network RF RadioFrequency RI Rank Indicator RRC Radio Resource Control RS ReferenceSignal RSRP Reference Signal Received Power RSRQ Reference SignalReceived Quality RSSI Received Signal Strength Indicator Rx Receive TFRETime/Frequency Resource Element Tx Transmit UMB Ultra-Mobile BroadbandUE User Equipment UL Uplink WCDMA Wideband Code Division Multiple AccessWiMax Worldwide Interoperability for Microwave Access

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of the example embodiments, as illustrated in theaccompanying drawings in which like reference characters refer to thesame parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe example embodiments.

FIG. 1 is an illustration of a transmission structure for a precodedspatial multiplexing mode in LTE;

FIG. 2A is an example illustration of a co-polarized antennaconfiguration;

FIG. 2B is an example illustration of a cross-polarized antennaconfiguration;

FIG. 3 is a depiction of codebook based precoding with a 2 Txcross-polarized antenna array;

FIG. 4A is an illustration of an antenna subelement;

FIG. 4B is an illustration of an antenna subelement featuring orthogonalpolarization compared to the subelement of FIG. 4A;

FIG. 4C is an illustration of an active antenna array;

FIG. 5A is an illustration of a 2 Tx active antenna array;

FIG. 5B is an illustration of a 8 Tx active antenna array;

FIG. 6 is an example illustration of message transmission, according tosome of the example embodiments;

FIG. 7 is an example node configuration of an eNodeB, according to someof the example embodiments;

FIG. 8 is an example node configuration of a user equipment, accordingto some of the example embodiments;

FIG. 9 is a flow diagram of example operations which may be taken by theeNodeB of FIG. 7, according to some of the example embodiments; and

FIG. 10 is a flow diagram of example operations which may be taken bythe user equipment of FIG. 8, according to some of the exampleembodiments.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particularcomponents, elements, techniques, etc. in order to provide a thoroughunderstanding of the example embodiments. However, the exampleembodiments may be practiced in other manners that depart from thesespecific details. In other instances, detailed descriptions ofwell-known methods and elements are omitted so as not to obscure thedescription of the example embodiments.

Some of the example embodiments presented herein are directed towardsproviding user equipment specific beamforming for downlinkcommunications. Such beamforming is not available with the use ofcurrent precoding techniques. As part of the development of the exampleembodiments presented herein, a problem will first be identified anddiscussed. The description below is organized as follows.

First, examples of existing precoders will be briefly discussed underthe sub-heading “Examples of Existing Precoder Codebooks for LTE”.Thereafter, an introduction on reference symbols which a user equipmentmay utilize for precoder determination is provided under the subheading“Channel State Information Reference Symbols (CSI-RS)”. Afterwards anintroduction of CoMP transmission is provided under the subheading“Coordinated Multipoint Transmission”. CoMP transmission may be used bythe user equipment to send signal quality measurements to the eNodeB.The eNodeB may in-turn provide the downlink communications based on, atleast in part, these measurements, according to some of the exampleembodiments. Thereafter, an introduction to antenna arrays and activeantennas is provided under the subheadings “Antenna Arrays” and “ActiveAntennas”, respectively. Afterwards, a detailed analysis of some of theexample embodiments will be provided.

Examples of Existing Precoder Codebooks for LTE

2 Tx Codebook

LTE Release-8, the first release of LTE, features the support codebookbased precoding for 2 antennas. Up to two layers can be transmitted(rank 1 and rank 2), thus making the precoder matrix W_(2×r) dimension2×1 and 2×2, respectively.

8 Tx Codebook

LTE Release-10, and later specifications, support a transmission modefor up to 8-layer spatial multiplexing for 8 Tx antennas using userequipment specific RS. Rank adaptation and possibly channel dependentprecoding is also supported. User equipment specific RS is used fordemodulation purposes and because of that the eNodeB is free to usewhatever precoder(s) it wants to, but it may be assisted in thedetermination of precoder(s) via CSI feedback from the user equipmentthat includes recommended precoder(s). For the time-frequency resourceof interest, the user equipment selects a precoder W_(8×r) out of a setof possible precoders in a precoder codebook which then is used togetherwith an input symbol vector s_(r×1) to produce an output signalx_(8×1)=W_(8×r)s_(r×1) for r layers.

Channel State Information Reference Symbols (CSI-RS)

In LTE Release-10, a new reference symbol sequence was introduced withthe intent of estimating channel state information, the CSI-RS. TheCSI-RS provides several advantages over basing the CSI feedback on thecommon reference symbols (CRS) which were used, for that purpose, inprevious releases. Firstly, the CSI-RS is not used for demodulation ofthe data signal, and thus does not require the same density (i.e., theoverhead of the CSI-RS is substantially less). Secondly, CSI-RS providesa much more flexible means to configure CSI feedback measurements (e.g.,which CSI-RS resources to measure on may be configured in a userequipment specific manner). Moreover, the support of antennaconfigurations larger than 4 antennas should resort to CSI-RS, since theCRS is only defined for at most 4 antennas.

Based on the CSI-RS, the user equipment may estimate the channel andconsequently also figure out which precoder suits the particularchannel. For the purpose of CSI feedback determination, the userequipment assumes that each of the rows corresponds to an antenna port(ports 15-22) on which a CSI-RS port signal is transmitted. The firstrow represents antenna port 15, the second row represents antenna port16, and so on. Each CSI-RS port signal is typically transmitted from anantenna of its own, meaning that there is a direct correspondencebetween a CSI-RS port and a physical antenna.

Coordinated Multipoint Transmission

Coordinated Multi Point (CoMP) transmission and reception refers to asystem where the transmission and/or reception at multiple,geographically separated antenna sites is coordinated in order toimprove system performance. The coordination can either be distributed,by means of direct communication between the different sites, or bymeans of a central coordinating node.

CoMP is a tool introduced in LTE to improve the coverage of high datarates, the cell-edge throughput and/or to increase system throughput. Inparticular, the goal is to distribute the user perceived performancemore evenly in the network by taking control of the inter-cellinterference.

CSI-RS Received Power

In order to enable efficient CoMP schemes, new forms of CSI feedback arepotentially needed. For instance it is an agreed working assumption, forLTE Rel-11, that the network can configure a user equipment to reportsignal qualities (e.g., received signal powers) based on measurements ona configured set of CSI-RS resources. A CSI-RS resource may loosely bedescribed as the pattern of resource elements on which a particularCSI-RS configuration is transmitted. A CSI-RS resource may be configuredthrough RRC signaling.

Such a measurement could be done coherently, in which case the userequipment needs to know the CSI-RS sequence that is transmitted on theCSI-RS resource, or incoherently in which case the transmitted actualsequence can be transparent to a user equipment. It should beappreciated that herein any signal quality measure based on measurementson CSI-RS resources is referred to as “CSI-RS received power”(CSI-RSRP), but it should be understood that CSI-RSRP encompasses anyquantity that represents a received quality of a CSI-RS signal. Theestimated values of the CSI-RSRP may then be fed back from the userequipment to the eNodeB.

Antenna Arrays

On the network side, base stations are often equipped with multipleantennas to be used for reception and transmission. The antennasintended for a transmission point (e.g., a cell, and/or a sector), forma so-called antenna array. Some typical antenna array constellations areillustrated in FIG. 2. For instance, one common antenna array layout isto use co-polarized antennas in order to construct antenna arrays asshown in FIG. 2( a). Furthermore, another common layout is to insteaduse cross-polarized antennas as shown in FIG. 2( b).

Using, for example, a 2 Tx cross-polarized antenna array (c.f. the topmost antenna setup in FIG. 2( b)) implies that the antenna array is fedwith two signals, x₁ and x₂. This is illustrated in FIG. 3 where it hasbeen assumed that a 2 Tx antenna array is used with codebook basedprecoding, so that the transmitted signal is x_(2×1)=W_(2×r)s_(r×1).

Active Antennas

An active antenna comprises a number of subelements that jointly formthe antenna. In FIG. 4( a) a subelement, in practice may be realized bya small physical device, as is illustrated. Each subelement will have apolarization direction which potentially can be orthogonal to anothersubelement's polarization. This is illustrated in FIG. 4( b) where asubelement with orthogonal polarization compared to the subelement inFIG. 4( a) is shown. Finally, in FIG. 4( c) an active antenna arraywhich consists of N_(S) subelements is shown. In general, but notnecessarily, all the subelements of an active antenna are of the samepolarization. Note that each given subelement i can be fed the givensignal x^((i)) not necessarily equal to x^((j)).

Herein, when dealing with more than one active antenna, the notationx_(i) ^((j)) will be utilized when referring to a signal, or function,related to the j:th subelement in the i:th antenna. These indexes willhowever be omitted when it is clear from the context what is beingreferred.

By combining two active antennas of different polarizations, asillustrated in FIG. 5( a), a 2 Tx antenna array may be created and fedwith two different signals, x₁ and x₂ where x_(i)[x_(i) ⁽¹⁾ . . . x_(i)^((N) ^(s) ⁾]^(T). Furthermore, by combining multiple 2 Tx antennaarrays, as illustrated in FIG. 5( b), an 8 Tx antenna array may becreated. Here the signals x_(i) ^((j)) are no longer explicitly shownbut they are still assumed to be present in the same manner as in FIG.5( a).

Existing precoder codebooks in different standards have been designedfor conventional antenna arrays. For example, in LTE Release 10 andbeyond, precoder codebooks for 2, 4 or 8 Tx antennas are supported.There is thus a precoder codebook suitable for each antenna array type.Hence, when using, for example, a 2 Tx antenna array, the standard isdesigned to use the 2 Tx codebook meaning that x₁ and x₂ can be fed tothe antenna array just as in FIG. 3.

Active antennas comprise many subelements and arrays of active antennascomprise even more. Such antenna setups were neither thought of, nortaken into account, when the existing codebooks were designed.Therefore, existing codebooks do not utilize the fact that thesubelements can be accessed as illustrated in FIG. 5.

Moreover, for large active antenna deployments, the sheer number ofsub-element antenna ports can create so many degrees of freedom that theCSI feedback overhead from a user equipment becomes prohibitive.

Active antennas, antenna arrays and arrays of active antennas may begeneralized to a system of multiple antennas. Herein, the phrasemultiple antenna system is used to describe a set of antennas(comprising one active antenna with multiple sub-elements) thatconstitutes a transmission point (i.e., with the intent to serve asufficiently isolated region of space, such as a cell and/or sector).

Another option is to base the determination of a beamformer in acorrelated domain by means of, for example, uplink measurements.Resorting to uplink measurements does however have several limitations.An example of such a limitation may be in deployments with downlinkinter-frequency carrier aggregation. In such a scenario, it is unlikelythat all terminals support the matching uplink carrier aggregation.Hence, for this scenario it will not be possible to perform uplinkmeasurements relevant for all carriers.

Another example of such a limitation is in FDD systems where the uplinkand downlink carriers are separated in frequency, and the measurementsmade in uplink may have reduced accuracy in the downlink. A furtherexample is an implementation disadvantage to introduce coupling betweenuplink and downlink processing.

The techniques described herein allow for feedback from the userequipment to be generated and used for guiding user equipment specificbeamforming in the azimuth and/or elevation domain by using multipleantenna systems even when existing codebooks not are designed for thispurpose.

The basic concept of the example embodiments is to configure a set ofCSI-RS resources, precoded in such a way that each CSI-RS span adirection within a correlated domain (i.e., a correlated vector space)of a multiple antenna system. By configuring a user equipment to measureand report the corresponding CSI-RSRP values to the network, the eNodeBcan use these values to improve the directivity within the correlateddomain of the transmission, such that the power transmitted alongdirections with poor signal quality is minimized, and power istransmitted along directions with good signal quality.

By utilizing the CSI-RSRP to acquire information about relativelystationary directions, the regular rapid PMI reporting can be offloadedto cover only the more dynamic variations in the channel, for which ithas been designed.

Overview of Beamforming According to the Example Embodiments

In this section, the example embodiments will be illustrated in moredetail by a number of examples. It should be noted that theseembodiments are not mutually exclusive. Components from one embodimentmay be tacitly assumed to be present in another embodiment and it shouldbe appreciated those components may be used in the other exampleembodiments.

Generating beamforming feedback from CSI-RSRP

In the following example embodiments the target is to illustrate howbeamforming feedback may be created by using CSI-RSRP measurements,potentially in combination with regular RI/PMI/CQI feedback.

Generating Elevation Beamforming Feedback from CSI-RSRP

Radio propagation channels tend to be highly correlated in the elevationdirection (i.e., the elevation domain), in particular for antennas thatare deployed over rooftops. Such correlation may result in almost allpower of the radio wave, which reaches a user equipment, typicallytraverses directions closely clustered in elevation domain.Specifically, typically all power is confined to one or two tightclusters in elevation.

In this example embodiment a set of CSI-RS signals are configured tospan different elevation directions. By configuring a user equipment tomeasure and report the corresponding CSI-RSRP values, the network (e.g.,eNodeB) may determine an elevation beamformer guided by reportedCSI-RSRP values, each corresponding to an associated elevation. Forexample, the eNodeB may select the elevation beamformer used for theCSI-RS with the highest reported CSI-RSRP.

FIG. 6 provides an example illustration of the communications which maybe transmitted between an eNodeB and a user equipment. First, the eNodeBmay transmit any number of CSI-RS reference signals to the userequipment (shown as circle 1). The eNodeB may configure the userequipment to measure and report the corresponding CSI-RSRP for at leasta subset of transmitted signals. Thereafter, the eNodeB may receive thecorresponding CSI-RSRP report for the user equipment (circle 2).Finally, the eNodeB may transmit data to the user equipment taking thereceived CSI-RSRP values into account in the selection of an elevationbeamformer that is used in subsequent data transmissions.

The communications illustrated in FIG. 6 will now be mathematicallydescribed. Consider the case with an N_(A) Tx active antenna array whereeach antenna comprises N_(s) subelements. A design featuring N_(Q)vectors yields the following:

${b_{q} = \begin{bmatrix}b_{1,q} \\b_{2,q} \\\vdots \\b_{N_{S},q}\end{bmatrix}},\mspace{14mu} {q = 1},\ldots \mspace{14mu},N_{Q}$

where b_(q) corresponds to a beamforming vector when applied to thesubelements of the i:th active antenna in the N_(A) Tx active antennaarray. It should be appreciated that the above representation may beextended to configurations with different number of subelements in eachantenna. Furthermore, it should be appreciated that the concept of asubelement is non-limiting in the sense that it may refer to anyvirtualization (e.g., linear mapping) of the physical antenna elements.For example, pairs of physical subelements may be fed the same signals,and hence share the same virtualized subelement antenna port.

The representation provided above will hence correspond to beamformingin the elevation assuming that the subelements are placed vertically,where each b_(q) typically corresponds to a targeted elevationdirection. The following beamformer matrix may thereafter be created:

${{B_{N_{A}N_{S} \times N_{A}}(q)} = \begin{bmatrix}b_{q} & 0 & \ldots & 0 \\0 & b_{q} & \; & \vdots \\\vdots & \; & \ddots & \; \\0 & \ldots & \; & b_{q}\end{bmatrix}},\mspace{14mu} {q = 1},\ldots \mspace{14mu},N$

The matrix presented above may be used to create the port to subelementmapping as

{tilde over (x)} _(N) _(A) _(N) _(S) _(×1)(q)=B _(N) _(A) _(N) _(S)_(×N) _(A) (q)x _(N) _(A) _(×1)(q)

where x_(N) _(A) _(×1)(q) is a CSI-RS signal for N_(A) ports. For thecreated output vector {tilde over (x)}_(N) _(A) N _(S) _(×1)(q) it isassumed that the first N_(s) values correspond to the subelements of oneactive antenna and that the second N_(s) values correspond to thesubelements of a second active antenna, etc. Hence, the equation abovedescribes a mapping from the N_(A) ports CSI-RS to the N_(A)N_(S)subelements. It should be appreciated that the structure herecorresponds to each active antenna in the array having the sameelevation beam. It is however possible to use a different elevation beamon each active antenna.

Due to the special structure in the beamformer matrixes, each index qcorresponds to a transmission in a certain direction defined by thebeamformers b₁, . . . , b_(N) _(Q) . Typically, the different CSI-RSsignals, x_(N) _(A) _(×1)(q), may be configured to be orthogonal, forexample, separated to different time/frequency resources.

Now, let the eNodeB be configured to transmit {tilde over (x)}_(N) _(A)_(N) _(S) _(×1)(q) for q=1, . . . , N_(Q). Furthermore, assume some userequipments are configured to report back the corresponding CSI-RSRPvalues meaning that one signal quality assessment, RSRP_(q) perreporting user equipment may be feedback for each transmitted {tildeover (x)}_(N) _(A) _(N) _(S) _(×1)(q), each corresponding to anassociated elevation beam. This will provide the eNodeB with usefulinformation regarding which elevation beamforming vector to choose forsubsequent transmissions (e.g., data transmissions) to a user equipment.

One potential choice for a beamforming vector may be for instance be tochoose the elevation beamforming vector, b_(q), corresponding to thestrongest RSRP_(q). The subsequent data transmission may further combinethe selected elevation beamformer with, for example, azimuth andinter-polarization beamforming which, may be based on a PMIrecommendation by the user equipment.

Alternatively, since the set of CSI-RS resources to measure RSRP on canbe configured UE-specifically, a UE can be configured to feed back onlythose RSRP values that are of relevance to a particular UE (i.e., thereis no need for a distant UE to feed back RSRP values for CSI-RSresources that are associated with very steep elevation angles).

Using CSI-RS with Fewer Ports than N_(A)

According to some of the example embodiments, the CSI-RSRP measurementsfor generating elevation beamforming feedback may be created from anysubset of N_(A) ports CSI-RS. For example, in common case of an N_(A) Txcross-polarized active antenna, with N_(A)/2 active antennas in eachpolarization, the beamformer matrix B_(N) _(A) _(N) _(S) _(×N) _(A) (q)may be represented as:

${B_{N_{A}N_{S} \times 2}(q)} = \begin{bmatrix}b_{q} & 0 \\\vdots & \vdots \\b_{q} & 0 \\0 & b_{q} \\\vdots & \vdots \\0 & b_{q}\end{bmatrix}$

where it is assumed that the first column will target the firstpolarization and the second column will target the other polarization inthe beamformer matrix. Applying the port to antenna mapping results inthe follow equation:

{tilde over (x)} _(N) _(A) _(N) _(S) _(×1)(q)=B _(N) _(A) _(N) _(S)_(×2)(q)X _(2×1)(q)

where x_(N) _(A) _(×1) is a two port CSI-RS which may enable elevationbeamforming feedback that may be utilized for at least one of thepolarizations. Hence, this will provide useful information for theeNodeB regarding which beamforming vector to choose for eachpolarization.

A more general case corresponds to adding a precoder, W, thatdistributes a r port CSI-RS pattern into N_(A) ports, such that:

{tilde over (x)} _(N) _(A) _(N) _(S) _(×1)(q)=B _(N) _(A) _(N) _(S)_(×N) _(A) (q)W _(N) _(A) _(×r) x _(r×1)(q),

where the above embodiment may be captured by configuring W as

${W_{N_{A} \times 2} = \begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}},$

where 0 and 1 are the vectors of all zeros and ones (with N_(A)/2elements), respectively. For instance, for CSI-RS resources used tomeasure elevation directions, it would be useful to try to spread theCSI-RS as evenly as possible in the azimuth direction, which correspondsto designing W to have as broad of an azimuth main lobe as possible.Typically, one would like to have the elevation lobe quite narrow andtherefore the beamforming vector may be designed accordingly. The lobein azimuth should on the other hand be quite wide and therefore thematrix W (which creates this lobe) may be designed accordingly. Such awide lobe may be achieved by, for example, spreading each port of aCSI-RS on all antennas, but with the phase rotations such that in notargeted direction the transmitted signals all destructively (orconstructively) combine. Another alternative, according to some of theexample embodiments, is to introduce azimuth phase variations overfrequency, in which case the beam, as seen over the entire bandwidth,may be very wide. Yet another alternative is to divide the transmitantennas in groups and transmit a specific CSI-RS port on antennas inone such group only (within a group of antennas the CSI-RS port can betransmitted with a wide lobe). This will have the benefit that therewill be no destructive/constructive combining between the differentgroups of antennas because different antenna ports of a CSI-RS are quasiorthogonal. It should be appreciated that W may also be made dependenton q, where there could be one beamformer for each CSI-RS.

Alternatively, for deployments with copolarized antennas, or for whichthere is a negligible risk of destructive combining over thepolarizations, (or the system bandwidth provides sufficientrandomization over frequency), a single port CSI-RS may be used,x_(1×1)(q), in combination with:

${B_{N_{A}N_{S} \times 1}(q)} = {\begin{bmatrix}b_{q} \\\vdots \\b_{q}\end{bmatrix}.}$

Furthermore, it should be appreciated that other beamformer matrixesB_(N) _(A) _(N) _(S) _(×k)(q) may be utilized in order to adapt to kports CSI-RS.

Generating Azimuth Beamforming Feedback from CSI-RSRP

In the above descriptions of the example embodiments, it is explainedhow CSI-RSRP may be utilized in order to generate elevation beamformingfeedback. The example embodiments presented herein however are notrestricted to be applied in the elevation domain. It should beappreciated that the example embodiments may be utilized in order tocreate beamforming feedback in any other domain or direction, forexample in an azimuth domain or direction.

Generating Joint Azimuth and Elevation Beamforming Feedback fromCSI-RSRP

It should also be appreciated that the example embodiments presentedherein may be used to generate joint elevation and beamforming feedback.Following the example steps provided under the subheading “GeneratingElevation Beamforming from CSI-RSRP”, the following representation ofN_(Q) beamforming vectors may be provided:

${b_{q} = \begin{bmatrix}b_{1,q} \\b_{2,q} \\\vdots \\b_{{N_{A}N_{S}},q}\end{bmatrix}},\mspace{14mu} {q = 1},\ldots \mspace{14mu},N_{Q}$

such that b_(q) corresponds to a polarization specific beamformingvector when applied to a cross polarized active antenna array (e.g.,that has its antenna elements and subelements positioned to span bothelevation and azimuth domain). This will hence potentially implybeamforming in both elevation and azimuth. Now consider the followingbeamforming matrix:

${{B_{N_{A}N_{S} \times 2}(q)} = \begin{bmatrix}b_{q} & 0 \\0 & b_{q}\end{bmatrix}},\mspace{14mu} {q = 1},\ldots \mspace{14mu},N$

where the two columns represent the two polarizations. Therefore, theport to subelement mapping may be provided as:

{tilde over (x)} _(N) _(A) _(N) _(S) _(×1)(q)=B _(N) _(A) _(N) _(S)_(×2)(q)x _(2×1)(q)

where it is assumed that x_(2×1)(q) is an CSI-RS for 2 ports. Hence,this will provide useful information for performing joint azimuth andelevation beamforming. It should be appreciated that the exampleembodiments described above may be extended to be used with copolarizedarrays, and/or to use CSI-RS with less or more than 2 ports, by forexample applying a precoder to the CSI-RS or by modifying B_(N) _(A)_(N) _(S) _(×r)(q) accordingly.

In summary, through a special configuration of CSI-RSRP andcorresponding measurements, it is possible to generate informationsuitable as feedback for elevation beamforming. Based on thisinformation the eNodeB may decide on elevation beamforming vectors touse for the active antennas.

It should be appreciated that the beamformer matrices B_(N) _(A) _(N)_(S) _(×N) _(A) may be extended to consider the case when differentactive antennas use different beamformer vectors b_(q). It should benoted that in the following we refer to a beamforming direction as aprimary pointing angle of a beamforming vector. Hence, a distinctbeamforming direction corresponds to using a beamformer with primarypointing angle in the distinct direction.

It should further be appreciated that each of the reference signals maybe transmitted in a distinct beamforming direction b_(n), however, thetransmitted data itself may be overlapping. It should also beappreciated that the reporting periodicity and/or mechanism is notessential for the example embodiments. The reporting may, for example,be configured so that a feedback report is triggered at a certainperiodicity. Alternatively, the feedback report may be provided by anevent based triggering that may be provided by the user equipment. Thus,the signal quality assessment, RSRP_(q), may be reported if a sufficientchange has incurred.

Example Node Configurations

FIG. 7 illustrates an example of an eNodeB 401 which may incorporatesome of the example embodiments discussed above. As shown in FIG. 7, theeNodeB 401 may comprise a radio circuitry 410 configured to receive andtransmit any form of communications or control signals within a network.It should be appreciated that the radio circuitry 410 may be comprisedas any number of transceiving, receiving, and/or transmitting units orcircuitry. It should further be appreciated that the radio circuitry 410may be in the form of any input/output communications port known in theart. The radio circuitry 410 may comprise RF circuitry and basebandprocessing circuitry (not shown).

The eNodeB 401 may further comprise at least one memory unit orcircuitry 430 that may be in communication with the radio circuitry 410.The memory 430 may be configured to store received or transmitted dataand/or executable program instructions. The memory 430 may also beconfigured to store any form of beamforming information, referencesignals, and/or feedback data or information. The memory 430 may be anysuitable type of computer readable memory and may be of volatile and/ornon-volatile type.

The eNodeB 401 may further comprise further comprises a networkinterface 440 and processing circuitry 420 which may be configured togenerate and analyze reference signals, and generate beamformedcommunications. The processing circuitry 420 may be any suitable type ofcomputation unit, e.g. a microprocessor, digital signal processor (DSP),field programmable gate array (FPGA), or application specific integratedcircuit (ASIC) or any other form of circuitry. It should be appreciatedthat the processing circuitry need not be provided as a single unit butmay be provided as any number of units or circuitry.

FIG. 8 illustrates an example of a user equipment 505 which mayincorporate some of the example embodiments discussed above. As shown inFIG. 8, the user equipment 505 may comprise radio circuitry 510configured to receive and transmit any form of communications or controlsignals within a network. It should be appreciated that the radiocircuitry 510 may be comprised as a single transceiving unit. It shouldbe appreciated that the radio circuitry 510 may be comprised as anynumber of transceiving, receiving, and/or transmitting units orcircuitry. It should further be appreciated that the radio circuitry 510may be in the form of any input/output communications port known in theart. The radio circuitry 410 may comprise RF circuitry and basebandprocessing circuitry (not shown).

The user equipment 505 may further comprise at least one memory unit orcircuitry 530 that may be in communication with the radio circuitry 510.The memory 530 may be configured to store received or transmitted dataand/or executable program instructions. The memory 530 may also beconfigured to store information relating to measured reference signals.The memory 530 may be any suitable type of computer readable memory andmay be of volatile and/or non-volatile type.

The user equipment 505 may also comprise processing circuitry 520 whichmay be configured to measure or analyze received reference signals. Theprocessing circuitry 520 may be any suitable type of computation unit,e.g. a microprocessor, digital signal processor (DSP), fieldprogrammable gate array (FPGA), or application specific integratedcircuit (ASIC) or any other form of circuitry. It should be appreciatedthat the processing circuitry need not be provided as a single unit butmay be provided as any number of units or circuitry.

Example Node Operations

FIG. 9 is a flow diagram depicting example operations which may be takenby the eNodeB 401 of FIG. 7 in the establishment of beamforming for amultiple antenna system. It should also be appreciated that FIG. 9comprises some operations which are illustrated with a darker boarderand some operations which are illustrated with a lighter boarder. Theoperations which are comprised in a darker boarder are operations whichare comprised in the broadest example embodiment. The operations whichare comprised in a lighter boarder are example embodiments which may becomprised in, or a part of, or are further operations which may be takenin addition to the operations of the boarder example embodiments. Itshould be appreciated that these operations need not be performed inorder. Furthermore, it should be appreciated that not all of theoperations need to be performed. The example operations may be performedin any order and in any combination.

Operation 10

The eNodeB 401 is configured to transmit 10, to a user equipment 505, aplurality of reference signals. Each reference signal is beamformed intoa distinct direction within at least one correlated domain of themultiple antenna system. The radio circuitry 410 is configured totransmit, to the user equipment, the plurality of reference signals.

According to some of the example embodiments, the plurality of referencesignals may be CSI-RS signals. According to some of the exampleembodiments, the at least one correlated domain may be an elevationand/or an azimuth domain.

Example Operation 12

According to some of the example embodiments, the transmitting 10 mayfurther comprise applying 12 a distinct beamforming vector to at leastone reference signal of the plurality of reference signals forsub-elements of at least one antenna. The applying is done such that theat least one reference signal is beamformed within the at least onecorrelated domain. The processing circuitry 420 is configured to applythe distinct beamforming vector to the at least one reference signal ofthe plurality of reference signals for sub-elements of at least oneantenna. It should be appreciated that non-limiting examples of applyinga beamforming vector for subelements of an antenna are mathematicallyexplained and provided under the subheading “Generating ElevationBeamforming Feedback from CSI-RSRP”.

According to some of the example embodiments, at least one referencesignal may be precoded onto a plurality of antenna elements, anddistributed onto sub-elements of at least one of the plurality ofantenna elements by the beamforming vector. According to some of theexample embodiments, a precoder may be constructed to have broadbeams(s) in a plane spanned by relative positions or multiple antennas.

According to some of the example embodiments, the precoder may map portsof the at least one reference signal into two groups. The two groups maycorrespond to two orthogonal polarizations of the multiple antennasystem. In such an embodiment, the at least one reference signal mayhave two ports. According to some of the example embodiments, theprecoding for the two orthogonal polarizations may be different.

According to some of the example embodiments, the precoder may befrequency selective.

Non-limiting examples of the use of a precoding matrix, W, are presentedherein under the subheading “Using CSI-RS with Fewer Ports than N_(A)”.It should be appreciated that the use of a precoder is not limited to ascenario where the CSI-RS has fewer ports than a number of antennas,N_(A).

According to some of the example embodiments the at least one referencesignal may have fewer ports than a number of antennas of the multipleantenna system. According to some of the example embodiments, the atleast one reference signal may have one port. A non-limiting descriptionof a reference signal comprising few ports than a number of antennas isprovided under the subheading “Using CSI-RS with Fewer Ports thanN_(A)”.

Operation 14

The eNodeB is further configured to receive 14, from the user equipment,signal quality assessments for each of the plurality of referencesignals. The radio circuitry 410 is configured to receive, from the userequipment, signal quality assessments for each of the plurality ofreference signals.

According to some of the example embodiments, at least one of the signalquality assessments may be a RSRP, RSRQ, or a RSSI. According to some ofthe example embodiments, the at least one of the signal qualityassessments may be comprised in a CQI feedback message.

Operation 16

The eNodeB is further configured to generate 16 downlink communicationsignals for antenna elements and/or subelements of the multiple antennasystem. The downlink communications signals are beamformed in atransmitting direction in the at least one correlated domain. Thetransmitting direction is determined, at least in part, from the signalquality assessments. The processing circuitry 420 is configured togenerate the downlink communications signals for antenna elements and/orsubelements of the multiple antenna system.

According to some of the example embodiments, the transmitting directionwithin the at least one correlated domain is associated with a highestmeasurement of the signal quality assessments. Non-limiting examples ofthe generation of downlink communication signals are provided under thesubheading “Generating Beamforming Feedback from CSI-RSRP”. It should beappreciated that the example embodiments presented herein are notlimited to the use of RSRP.

FIG. 10 is a flow diagram illustrating example operations that may betaken by the user equipment 505 of FIG. 8 in the establishment ofbeamforming for a multiple antenna system. It should also be appreciatedthat FIG. 10 comprises some operations which are illustrated with adarker boarder and some operations which are illustrated with a lighterboarder. The operations which are comprised in a darker boarder areoperations which are comprised in the broadest example embodiment. Theoperations which are comprised in a lighter boarder are exampleembodiments which may be comprised in, or a part of, or are furtheroperations which may be taken in addition to the operations of theboarder example embodiments. It should be appreciated that theseoperations need not be performed in order. Furthermore, it should beappreciated that not all of the operations need to be performed. Theexample operations may be performed in any order and in any combination.

Operation 20

The user equipment 505 is configured to receive 20, from an eNodeB, aplurality of reference signals. Each reference signal is beamformed intoa distinct direction within at least one correlated domain of themultiple antenna system. The radio circuitry 510 is configured toreceive, from the eNodeB, the plurality of reference signals.

According to some of the example embodiments, the plurality of referencesignals are CSI-RS signals. According to some of the exampleembodiments, the at least one correlated domain is an elevation and/oran azimuth domain.

Operation 22

The user equipment 505 is further configured to transmit 22, to theeNodeB, signal quality assessments of each of the plurality of referencesignals. The radio circuitry 510 is configured to transmit, to theeNodeB, signal quality assessments of each of the plurality of referencesignals. It should be appreciated that the processing circuitry 520 maybe configured to perform measurements in providing such signal qualityassessments. It should further be appreciated that the manner in whichsuch measurements may be performed may be provided by means of aconfiguration provided by the eNodeB.

According to some of the example embodiments, at least one of the signalquality assessments is a RSRP, RSRQ, or a RSSI. According to some of theexample embodiments, the signal quality assessments may be comprised ina CQI feedback messaging.

Operation 24

The user equipment 505 is further configured to receive 24, from theeNodeB, downlink communications. The downlink communications arebeamformed in a receiving direction in the at least one correlateddomain. The receiving direction is determined, at least in part, fromthe signal quality assessments. The radio circuitry 510 is configured toreceive, from the eNodeB, the downlink communications.

According to some of the example embodiments, the receiving directionwithin the at least one correlated domain may be associated with ahighest measurement of the signal quality assessment. Specifically, thereceiving direction may be based on a highest measurement based on atleast one of a RSRP, RSRQ, or a RSSI.

CONCLUSION

It should be noted that although terminology from 3GPP LTE has been usedherein to explain the example embodiments, this should not be seen aslimiting the scope of the example embodiments to only the aforementionedsystem. Other wireless systems, including WCDMA, WiMax, UMB and GSM, mayalso benefit from the example embodiments disclosed herein.

Also note that terminology such as eNodeB and user equipment should beconsidered as non-limiting and does in particular not imply a certainhierarchical relation between the two. In general “eNodeB” could beconsidered as device 1 and “user equipment” as device 2, and these twodevices communicate with each other over some radio channel.Furthermore, while the example embodiments focus on wirelesstransmissions in the downlink, it should be appreciated that the exampleembodiments are equally applicable in the uplink.

The description of the example embodiments provided herein have beenpresented for purposes of illustration. The description is not intendedto be exhaustive or to limit example embodiments to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of various alternativesto the provided embodiments. The examples discussed herein were chosenand described in order to explain the principles and the nature ofvarious example embodiments and its practical application to enable oneskilled in the art to utilize the example embodiments in various mannersand with various modifications as are suited to the particular usecontemplated. The features of the embodiments described herein may becombined in all possible combinations of methods, apparatus, modules,systems, and computer program products. It should be appreciated thatthe example embodiments presented herein may be practiced in anycombination with each other.

It should be noted that the word “comprising” does not necessarilyexclude the presence of other elements or steps than those listed andthe words “a” or “an” preceding an element do not exclude the presenceof a plurality of such elements. It should further be noted that anyreference signs do not limit the scope of the claims, that the exampleembodiments may be implemented at least in part by means of bothhardware and software, and that several “means”, “units” or “devices”may be represented by the same item of hardware.

A “device” as the term may be used herein, is to be broadly interpretedto include a radiotelephone having ability for Internet/intranet access,web browser, organizer, calendar, a camera (e.g., video and/or stillimage camera), a sound recorder (e.g., a microphone), and/or globalpositioning system (GPS) receiver; a personal communications system(PCS) user equipment that may combine a cellular radiotelephone withdata processing; a personal digital assistant (PDA) that can include aradiotelephone or wireless communication system; a laptop; a camera(e.g., video and/or still image camera) having communication ability;and any other computation or communication device capable oftransceiving, such as a personal computer, a home entertainment system,a television, etc. Furthermore, a device may be interpreted as anynumber of antennas or antenna elements.

Although the description is mainly given for a user equipment, asmeasuring or recording unit, it should be understood by the skilled inthe art that “user equipment” is a non-limiting term which means anywireless device, terminal, or node capable of receiving in DL andtransmitting in UL (e.g. PDA, laptop, mobile, sensor, fixed relay,mobile relay or even a radio base station, e.g. femto base station).

A cell is associated with a radio node, where a radio node or radionetwork node or eNodeB used interchangeably in the example embodimentdescription, comprises in a general sense any node transmitting radiosignals used for measurements, e.g., eNodeB, macro/micro/pico basestation, home eNodeB, relay, beacon device, or repeater. A radio nodeherein may comprise a radio node operating in one or more frequencies orfrequency bands. It may be a radio node capable of CA. It may also be asingle- or multi-RAT node. A multi-RAT node may comprise a node withco-located RATs or supporting multi-standard radio (MSR) or a mixedradio node.

The various example embodiments described herein are described in thegeneral context of method steps or processes, which may be implementedin one aspect by a computer program product, embodied in acomputer-readable medium, including computer-executable instructions,such as program code, executed by computers in networked environments. Acomputer-readable medium may include removable and non-removable storagedevices including, but not limited to, Read Only Memory (ROM), RandomAccess Memory (RAM), compact discs (CDs), digital versatile discs (DVD),etc. Generally, program modules may include routines, programs, objects,components, data structures, etc. that perform particular tasks orimplement particular abstract data types. Computer-executableinstructions, associated data structures, and program modules representexamples of program code for executing steps of the methods disclosedherein. The particular sequence of such executable instructions orassociated data structures represents examples of corresponding acts forimplementing the functions described in such steps or processes.

In the drawings and specification, there have been disclosed exemplaryembodiments. However, many variations and modifications can be made tothese embodiments. Accordingly, although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation, the scope of the embodiments being defined bythe following claims.

1. A method, in an eNodeB, for establishing beamforming for a multipleantenna system, the eNodeB being comprised in a wireless communicationsnetwork, the method comprising: transmitting, to a user equipment, aplurality of reference signals, each reference signal being beamformedinto a distinct direction within at least one correlated domain of themultiple antenna system; receiving, from the user equipment, signalquality assessments for each of the plurality of reference signals; andgenerating downlink communication signals for antenna elements and/orsub-elements of the multiple antenna system, the downlink communicationsignals being beamformed in a transmitting direction in the at least onecorrelated domain, said transmitting direction being determined,in-part, from the signal quality assessments.
 2. The method of claim 1,wherein at least one of the signal quality assessments is a referencesignal received power, RSRP, a reference signal received quality, RSRQ,or a received signal strength indicator, RSSI.
 3. The method of claim 1,wherein the plurality of reference signals are Channel State InformationReference Symbols, CSI-RS.
 4. The method of claim 1, wherein thetransmitting direction within the at least one correlated domain isassociated with a highest measurement of the signal quality assessments.5. The method of claim 1, wherein the at least one correlated domain isan elevation and/or azimuth domain.
 6. The method of claim 1, whereinthe transmitting further comprises applying a distinct beamformingvector to at least one reference signal of the plurality of referencesignals, for sub-elements of at least one antenna, such that the atleast one reference signal is beamformed within the at least onecorrelated domain.
 7. The method of claim 6, wherein the at least onereference signal is precoded onto a plurality of antenna elements, anddistributed onto sub-elements of at least, one of said plurality ofantenna elements by the beamforming vector.
 8. The method of claim 7,wherein said at least one reference signal has fewer ports than a numberof antennas of the multiple antenna system.
 9. The method of claim 8,wherein said at least one reference signal has one port.
 10. The methodof claim 8, wherein a precoder is constructed to have broad beam(s) in aplane spanned by relative positions or multiple antennas.
 11. The methodof claim 7, wherein said precoder maps ports of the at least onereference signal into two groups, said two groups corresponding to twoorthogonal polarizations of the multiple antenna system.
 12. The methodof claim 11, where said at least one reference signal has two ports. 13.The method of claim 11, where precoding for the two orthogonalpolarizations is different.
 14. The method of claim 7, wherein saidprecoder is frequency selective.
 15. The method of claim 1, wherein atleast one of the signal quality assessments are comprised in a ChannelQuality Indicator, CQI, feedback message.
 16. An eNodeB for establishingbeamforming for a multiple antenna system, the eNodeB being comprised ina wireless communications network, the eNodeB comprising: radiocircuitry configured to transmit, to a user equipment, a plurality ofreference signals, each reference signal being beamformed into adistinct direction within at least one correlated domain of the multipleantenna system; the radio circuitry further configured to receive, fromthe user equipment, signal quality assessments for each of the pluralityof reference signals; and processing circuitry configured to generatedownlink communication signals for antenna elements and/or sub-elementsof the multiple antenna system, the downlink communication signals beingbeamformed in a transmitting direction in the last one correlateddomain, said transmitting direction being determined, in-part, from thesignal quality assessments.
 17. The eNodeB of claim 16, wherein at leastone of the signal quality assessments is a reference signal receivedpower, RSRP, a reference signal received quality, RSRQ, or a receivedsignal strength indicator, RSSI.
 18. The eNodeB of claim 16, wherein theplurality of reference signals are Channel State Information ReferenceSymbols, CSI-RS.
 19. The eNodeB of claim 16, wherein the transmittingdirection within the at least one correlated domain is associated with ahighest measurement of the signal quality assessment.
 20. The eNodeB ofclaim 16, wherein the at least one correlated domain is an elevationand/or azimuth domain.
 21. The eNodeB of claim 16, wherein the radiocircuitry is further configured to apply a distinct beamforming vectorto at least one reference signal of the plurality of reference signals,for sub-elements of at least one antenna, such that the at least onereference signal is beamformed in within the at least one correlateddomain.
 22. The eNodeB of claim 21, wherein the at least one referencesignal is precoded onto a plurality or antenna elements, and distributedonto sub-elements of at least one of said plurality of antenna elementsby the beamforming vector.
 23. The eNodeB of claim 22, wherein said atleast one reference signal has fewer ports than a number of antennas ofthe multiple antenna system.
 24. The eNodeB of claim 23, wherein said atleast one reference signal has one port.
 25. The eNodeB of claim 23,wherein a precoder is constructed to have broad beam(s) in a planespanned by relative positions or multiple antennas.
 26. The eNodeB ofclaim 22, wherein said precoder maps ports of the at least one referencesignal into two groups, said two groups corresponding to two orthogonalpolarizations of the multiple antenna system.
 27. The eNodeB of claim26, where said at least one reference signal has two ports.
 28. TheeNodeB of claim 26, where precoding for the two orthogonal polarizationsis different.
 29. The eNodeB of claim 22, wherein said precoder isfrequency selective.
 30. The eNodeB of claim 16, wherein the signalquality assessments are comprised in a Channel Quality Indicator, CQI,feedback messaging.
 31. A method, in a user equipment, for establishingbeamforming for a multiple antenna system, the user equipment beingcomprised in a wireless communications network, the method comprising:receiving, from an eNodeB, a plurality of reference signals, eachreference signal being beamformed into a distinct direction within atleast one correlated domain of the multiple antenna system;transmitting, to the eNodeB, signal quality assessments of each of theplurality of reference signals; and receiving, from the eNodeB, downlinkcommunications, said downlink communications being beamformed in areceiving direction in the at least one correlated domain, saidreceiving direction being determined, in-part, from the signal qualityassessments.
 32. The method of claim 31, wherein at least one of thesignal quality assessments is a reference signal received power, RSRP, areference signal received quality, RSRQ, or a received signal strengthindicator, RSSI.
 33. The method of claim 31, wherein the plurality ofreference signals are Channel State Information Reference Symbols,CSI-RS.
 34. The method of claim 31, wherein the receiving directionwithin the at least one correlated domain is associated with a highestmeasurement of the signal quality assessment.
 35. The method of claim31, wherein the at least one correlated domain is an elevation and/orazimuth domain.
 36. The method of claim 31, wherein the signal qualityassessments are comprised in a Channel Quality Indicator, CQI, feedbackmessaging.
 37. A user equipment for establishing beamforming for amultiple antenna system, the user equipment being comprised in awireless communications network, the user equipment comprising: radiocircuitry configured to receive, from an eNodeB, a plurality ofreference signals, each reference signal being beamformed into adistinct direction within at least one correlated domain of the multipleantenna system; the radio circuitry further configured to transmit, tothe eNodeB, signal quality assessments of each of the plurality ofreference signals; and the radio circuitry further configured toreceive, from the eNodeB, downlink communications, said downlinkcommunications being beamformed in a receiving direction in the last onecorrelated domain, said receiving direction being determined, in-part,from the signal quality assessments.
 38. The user equipment of claim 37,wherein at least one of the signal quality assessments is a referencesignal received power, RSRP, a reference signal received quality, RSRQ,or a received signal strength indicator, RSSI.
 39. The user equipment ofclaim 37, wherein the plurality of reference signals are Channel StateInformation Reference Symbols, CSI-RS.
 40. The user equipment of claim37, wherein the receiving direction within the at least one correlateddomain is associated with a highest measurement of the signal qualityassessment.
 41. The user equipment of claim 37, wherein the at least onecorrelated domain is an elevation and/or azimuth domain.
 42. The userequipment of claim 37, wherein the signal quality assessments arecomprised Channel Quality Indicator, CQI, feedback messaging.